I do work professionally as a "plant doctor" and I do routinely make many house in NYC.
This is a great example of the problems associated with unnecessary repotting, the single most common cause of indoor plant problems. Aspidistra, somewhat ironically called Cast Iron Plant, are slow growers that thrive on neglect. They do not tolerate direct sunlight or soil that stays constantly damp. The hard dry soil in the original rootball suited this plant just fine. The soil you added has kept the soil around the roots too damp and the roots have been slowly dying from root rot. Dying roots are not able to absorb water sufficiently for the foliage which starts to discolor and die.
Fertilizer is intended for use on healthy plants that are growing vigorously. It is NOT medicine and should never be used as cure for ailing plants. I wish fertilizer was the simple answer for you, but it is not.
You have two options. One is to remove all excess soil you added to the top of the original rootball and then allow the soil to become almost bone dry between waterings. Add just enough water so that it dries out again in about a week. Perhaps there are enough roots remaining and new roots will gradually emerge provided you water only in small quantities so that the soil dries out every week or so.
The second approach is to carefully remove the plant from its pot. Then, gently remove all excess soil that is not in immediate contact with healthy roots. Health roots will be firm and wiry. Damaged roots will be shriveled, soft, dark and mushy and should be removed. If you have some healthy roots remaining, move your Aspidistra to the SMALLEST pot that will accommodate the remaining roots and just enough soil to barely cover them. The goal here is to surround the roots with just enough soil to retain moisture for up to a week, but no more. The roots require oxygen just as much as they need moisture. Use a porous potting mix of 3 parts peat moss and 1 part perlite.
Your plant is in poor condition and may not make it. At best, the recovery will be very slow and will require patience and tolerance of its minimal needs on your part. Resist the temptation to nurture this plant.
Good luck! Let me know if you have further questions.
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